L4701 HST 'stumble' under load

[stuckytough]

I just moved from an L3400 gear drive to an L4701 HST. This is my first HST so just learning how it 'should' function.

The L4701 is used, under 400 hrs and operates well. When I'm under load, I notice a couple quirks with the hydraulic system

1. under load - either turning fast, or with a heavy bucket -> when i turn the steering wheel i get a 'stumble' or grumble at what appears to be a high load point. I get this feedback through my hands/ steering wheel as a vibration or 'stumble'.. almost like I'm hitting a limit or regulator threshold.. ?

2. when driving the tractor straight and going over hills (up and down), i notice a considerable change in sound (almost a gear rattle type sound) when i transition from 0 load to positive load (uphill) or negative load (downhill). i really notice it when rolling through small hills and transitioning between uphill and downhill.

I drove a brand new tractor at the dealer and did not experience either of these..
Are these symptoms 'normal wear' for a tractor with under 400 hours. anything i should be concerned about? any adjustment on actions i can take to correct?

Last question - I like to do all of my own maintenance, but am a HST novice. Is there HST 101 training material someone? Id like to start with the basics: is the HST pump the same pump that operates the loader (or separate)? is there a hydro circuit diagram? common failures / maintenance procedures beyond fluid and filter changes?

Sorry - multiple questions. Thanks,

 

[NCL4701]

Somewhat apologize for answering your question with a question, but the more I think about it the more I wonder: what engine RPM are you typically running during the described behaviors?

1) Steering grumble/stumble: As I understand it, there’s no mechanical connection between the steering wheel and front wheels; just hydraulic cylinder moving the steering knuckles. I have not noticed a delay, hesitation, or inability to turn the wheels under any circumstance so far. I have noticed a little grumbly feedback with: heavy load on loader, hard ground/pavement, stationary or creeping. Low engine RPM seems to contribute to the issue. High engine RPM seems to eliminate or almost eliminate the issue. Fast steering, never noticed it but a lot of times if I’m really steering as fast as possible I tend to throw the wheel and catch it when I want it to stop so maybe I don’t notice.

2) Gear noise: The HST transmission makes a whining noise when pulling. The harder the pull, the more the whine. So there is a change in transmission noise from downhill to flat to uphill, all other things being equal. The higher the gear range and lower the engine RPM the more noticeable it is. Not sure if increased RPM truly reduces transmission noise or if it’s just a perceived reduction due to increased engine noise. So far as a true “dry gear” type noise, not familiar with that.

When I got mine everything I read and multiple people at dealer advised to keep RPM up and adjust speed with gear range and HST pedal. Dealer suggested 1800 RPM to 2000RPM as a minimum range for routine operation and made it clear wide open throttle for long periods of time was NOT abuse but what it was made to do. Coming from thousands of hours on old gas tractors I felt like I was abusing it at 2000RPM, much less WOT. However, that’s how it’s designed to be run.

Point of all that is unless fluid levels are low such that you have some real gear noise (unlikely but possible) it doesn’t sound like it has any real “problems”. It sounds like you’re running with engine RPM lower than what it’s designed to operate with and have some mild symptoms consistent with low hydraulic flow. Increase RPM to 1800 minimum WOT max will likely straighten it out.

I don’t know what RPM you’re normally running while working it, so take that for what it’s worth which may be nothing.

You can download a Workshop Manual here:http://kubotabooks.com/AutoIndex/in...s/&AutoIndex=54d6e5b9534ec1e7a5829379813886b4

That will give you some more detailed info on the internal workings of the machine and it’s a good thing to have regardless.

Some general info. Two hydraulic pump: one power steering; one everything else (HST, loader, 3 point, rear remotes). The HST is basically a variable displacement hydraulic piston motor which uses a swash plate to control speed and direction of rotation. The HST pedal controls the swash plate position. If you want to understand how that works, there are some good videos on YouTube. Look for variable displacement hydraulic motor or pump (they’re basically the same just depending on how they’re hooked up). That hydraulic motor feeds into a 3 speed gear box and at that point it’s conventional gears out to the wheels.

Like most hydraulic motors, the cooling for the HST is the flow of fluid through the motor. So the general rule IMO is lower gear, more pedal to achieve desired speed. Personally, I don’t get all wound up about it, but running the loader into a pile repeatedly and slowly maneuvering in high range is not advisable. The need for flow is also the reason for keeping engine RPM up during use. The DPF likes the higher RPM and so does the HST. The engine is made to run the higher RPM constantly so run it.

I have zero regrets with mine and had many hours on old gas tractors before getting the Kubota. It sort of looks like the tractors I had experience with but it is a VERY different machine. And it’s not just diesel v gas. This isn’t your grandpa’s diesel. What keeps a Tier 4 HST happy is quite different from what keeps a gear drive old gasser or old diesel happy.

One last thing that’s kind of counterintuitive: if you’re pulling something and running out of torque of course the rules still hold to shift to a lower range and increase throttle BUT instead of giving it more HST pedal, give it less. You’ll actually get a bit more torque with less pedal.

 

[stuckytough]

Thanks for the detailed reply. The power steering feedback is much more noticeable at lower rpms.

Reading your response - I definitely have 'old school' habits with respect to engine rpm. my tendency is to only run the rpm I need for the job. like you said, i feel running at high idle for low load tasks seems like a waste of fuel and addition wear (engine revolutions) that might not be necessary.

... i need to change my mindset and get calibrated to modern HST operating requirements.

Thanks again. I will do some higher rpm testing over the next couple days.

 

[NCL4701]

One thing I’ve found with mine in regard to fuel wasting, which concerned me as well, is unlike a fully mechanical diesel fuel consumption is much more associated with load than with RPM. For example, I can run at 540 PTO RPM running a rotary cutter and use very little fuel if mowing scraggly crap that takes little torque. Doing the once a year cutting of sawgrass by the swampy creek area, same RPM but the engine sound changes noticeably and it starts drinking fuel like a camel at a watering hole even though it’s not bogging down. It took me probably around 50 hours of seat time to get my mind right even though I knew intellectually I was treating it right. Initially it just felt wrong.

Hope you enjoy yours as much as I’ve enjoyed mine.

 

[RCW]

Your hydraulic pump serves both the hydraulic system (loader, 3 point hitch), as well as the power/hydrostatic steering. Some pumps are in two parts, but your tractor doesn't have a separate power steering pump like we'd have on our cars or trucks.

I've noticed the same with my little BX. Sometimes the power steering feels "starved" momentarily under similar circumstances. I can feel it in the wheel, just like you.

I think there's just more demand for other hydraulic applications, and it's just a fleeting moment...not a long duration condition.

I don't think it's an issue with your tractor, but more so the design of the system.